Cells dissociated from a variety of tissues such as the mammary gland, the pancreatic islet, the lung or liver have been demonstrated to be capable under appropriate experimental conditions to self-assemble into multicellular aggregates with organization and architecture which resembles their original tissue. The ability to understand and control the morphogenesis of these three dimensional tissue-like structures is a fundamental objective of cell and developmental biology and tissue engineering research.
In the field of liver tissue engineering, self-assembled spheroidal aggregates of isolated primary hepatocytes have been obtained in suspension culture or by utilizing moderately-adhesive substrata of natural or artificial extracellular matrices such as laminin, fibronectin or collagen I. The extracellular matrix proteins mentioned above may be conjugated with cell adhesion peptides, such as Arg-Gly-Asp (RGD) and Tyr-Ile-Gly-Ser-Arg (YIGSR; SEQ ID NO:1)). However, hepatocytes adhere tightly to these substrata, which results in extended and spread cell morphology, and low levels of liver-specific activities which may be a result of hepatocyte de-differentiation.
In contrast, if hepatocytes are anchored too loosely to the substrata, hepatocyteaggregate into spheroids which form in vivo-like 3D architecture and which exhibit tissue-like cell-cell and cell-matrix connectivity and enhanced liver-specific activities, membrane polarities and liver ultrastructure, such as bile canaliculi, tight junctions and gap junctions. The features of hepatocyte spheroids suggest their potential applications in bioartificial liver aided devices (BLAD) and drug metabolic/hepatotoxicity studies.
However, the usefulness of 3D hepatocyte spheroids is limited due to the poor mass transport of nutrients, oxygen, xenobiotics and metabolites into and from the core of these large cellular aggregates. Cell loss is also a critical issue in forming and maintaining these spheroids due to the poor adhesion of spheroids on the substratum. Accordingly, an alternative to producing hepatocyte spheroids which offers some of the advantageous properties of spheroids but avoids some of the disadvantages of spheroids is desirable.